A map matching process in a navigation system basically estimates a position of a subject vehicle by, for example, acquiring a position data from a GPS satellite and combining the position data with additional data from a gyro and a speed sensor, and then determines the position of the subject vehicle by matching an estimated position to road data in a road map database. In this case, the matching process suffers from closely located roads in a small area or errors of sensing devices in terms of correctness and accuracy. That is, the current position of the vehicle may possibly be wrongly determined. In addition, the wrongly determined current position may affect a condition of vehicle control that uses current position data for suitably controlling the vehicle. The wrongly determined control condition may causes uncomfortableness, uncertainty or the like for a user of the vehicle.
A technique disclosed in Japanese Patent Document JP-A-2002-254980 uses map matching reliability index for evaluating accuracy of a vehicle position that is determined by using the map matching process, and controls a front light of the vehicle based on the evaluation of the positional accuracy. The reliability index has a predetermined set of ratings, for example, for determining reliability of map matching situations such as a turn case, a curve case, an off-the-route case or the like at an intersection, a branch, a corner or other points in a road. The reliability index has higher values in the situations where the current position of the vehicle is modified based on the estimated position of the vehicle. That is, the reliability index increases when the current position of the vehicle in the map is modified based on the estimation.
On the other hand, the road on the map data determined by using the map matching process has an increased possibility of errors when, for example, two roads starting from an intersection make a very small angle between them. That is, the vehicle position on the map just after passing the intersection may very well be wrong. Therefore, determining the reliability of the map matching process by utilizing the map data that specifies the two adjacent roads before passing the intersection is effective and useful for avoiding the erroneous control of the vehicle or the like.
However, the technique described above does not include consideration regarding a prior determination of erroneous map matching, that is, there is no consideration of a possibility of errors in the map matching.